Bump nth-check and rollup-plugin-postcss

Bumps [nth-check](https://github.com/fb55/nth-check) and [rollup-plugin-postcss](https://github.com/egoist/rollup-plugin-postcss). These dependencies needed to be updated together.

Updates `nth-check` from 1.0.2 to 2.1.1
- [Release notes](https://github.com/fb55/nth-check/releases)
- [Commits](fb55/nth-check@v1.0.2...v2.1.1)

Updates `rollup-plugin-postcss` from 4.0.0 to 4.0.2
- [Release notes](https://github.com/egoist/rollup-plugin-postcss/releases)
- [Changelog](https://github.com/egoist/rollup-plugin-postcss/blob/master/CHANGELOG-OLD.md)
- [Commits](egoist/rollup-plugin-postcss@v4.0.0...v4.0.2)

---
updated-dependencies:
- dependency-name: nth-check
  dependency-type: indirect
- dependency-name: rollup-plugin-postcss
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>